The present invention relates to an optical recording and reproducing system for optically recording an information signal in a disk and optically reproducing the information signal from the disk, and more particularly to an access control circuit for such an optical recording and reproducing system suitable for high-speed access.
As the conventional optical recording and reproducing system is known one which is described by JP-A-59-207439 and has a construction shown in FIG. 11. This first prior art will now be explained.
A stroke signal 225 supplied from an external control circuit (not shown) is preset into a counter 217. The stroke signal 225 indicates the movement stroke of an optical head 201. An output signal of the counter 217 is supplied to a reference velocity generating circuit 222 and a control circuit 223. The control circuit 223 is supplied with a head position signal 230 from a head position detector 213 in addition to the output signal of the counter 217. During a period of time from the instant of time when the output signal of the counter 217 is zero and the head position signal 230 becomes zero until a predetermined time lapses (i.e. during a period of time from the arrival of the optical head to a target or desired position until the lapse of the predetermined time), the control circuit 223 supplies, to a second switch 228 and a third switch 238, signals for connecting these switches to their terminal A sides shown in FIG. 11. When the second switch 228 is connected to the A side, a lens position signal 226 detected by a lens position detector 206 and indicative of the position (in a tracking direction indicated by arrow 231) of a lens movable part 208 for an optical base 205 in the optical head 201 is fed back to a lens actuator 207 through a second filter circuit 227, the second switch 228 and a second power amplifier 229. As a result, the lens movable part 208 is located into a position at which the lens position signal 226 is zero. On the other hand, the third switch 238 connects an output signal of the first switch 220 to a first amplifier 221. At this time, a position of the lens movable part 208 in a focus direction is detected by a focus error detector 203. The detected focus error signal 214 is fed back to a focus actuator 204 through a focus control circuit 202. Thereby, a focus control is made so that a recording surface of a disk 210 is always in a focal position of an objective lens. The disk 210 is rotated by an output shaft 601 of a motor 600 which in turn is driven by a drive circuit (not shown).
Until the head position signal 230 becomes smaller than a predetermined value with the output signal of the counter 217 being zero (i.e. until the optical head 201 reaches a position which is in front of the target position by a predetermined distance therefrom), the control circuit 223 outputs to the first switch 220 a signal for connecting the switch to its terminal B side. Thus, a reference velocity signal generated from the reference velocity signal generating circuit 222 in accordance with the output signal of the counter 217 is supplied to a first power amplifier 221 through a differential amplifier circuit 219 and the first switch 220. A head actuator 212 is driven in accordance with the reference velocity signal. When the head actuator 212 is thus driven, the optical head 201 mechanically connected to the head actuator 212 by a shaft 612 is similarly driven toward the target position. The movement of the optical head 201 at this time is detected by the head position detector 213 which in turn supplies the head position signal 230 to a velocity detector 218. The velocity detector 218 detects the velocity of the optical head 201 from the head position signal 230. The detected head velocity signal is fed back to the differential amplifier circuit 219. In this manner, a velocity control is made so that the velocity of the optical head 201 follows a velocity indicated by the reference velocity signal.
The head position signal 230 is also supplied to a direction pulse generating circuit 215 which in turn generates a direction pulse corresponding to the direction of movement of the optical head 201 each time the optical head 201 moves over a predetermined distance. This direction pulse is fed back to the counter 217 which in turn makes a subtraction for the preset stroke signal. The reference velocity signal generated by the reference velocity signal generating circuit 222 supplied with the output signal of the counter 217 becomes small as the optical head 201 gets near the target position. Thus, a velocity control for the optical head 201 is made in such a manner that the optical head is first accelerated with a large acceleration and thereafter follows the reference velocity when the velocity of the head reaches the reference velocity. Generally, the deceleration of the reference velocity until the target position is established to a large value nearly equal to the value of acceleration in order to realize high-speed track access. The lens movable part 208 including the objective lens is located in a position at which the output signal 226 of the lens position detector 206 is zero. Therefore, the lens movable part 208 is not almost moved even for the large acceleration and deceleration of the optical head 201 by the velocity control. Accordingly, a positional error of the objective lens is suppressed to a very small value.
When the optical 201 reaches the position in front of the target position distanced by the predetermined distance under the above-mentioned velocity control, the control circuit 223 outputs to the first switch 220 a signal for connecting the switch to its terminal A side. Thereby, the head position signal 230 detected by the head position detector 213 indicative of the position of the optical head 201 is supplied to the first power amplifier 221 through a first filter circuit 216 for phase compensation and the first switch 220. On the basis of the head position signal 230, the head actuator 212 is driven to make a position control for the optical head 201 so that the head position signal 230 becomes zero. As a result, the optical head 201 is located into the target position. At this time, the lens movable part 208 including the objective lens is located into a position at which the output signal 226 of the lens position detector 206 is zero, like the case of the velocity control for the optical head. Therefore, even if the optical head 201 is located into the target position by a large deceleration, a positional deviation of the lens movable part 208 is not almost produced. A (decreasing) vibration of the lens movable part 208 caused by a positional deviation after the optical head 201 has been located is not generated and hence the access time can be shortened. Further, it is possible to eliminate any bad influence on the optical system resulting from a large positional deviation of the objective lens in the optical head 201.
After the optical head 201 has been thus located into the target position, the control circuit 223 outputs, to the second switch 228 and the third switch 238, signals for connecting these switches to their terminal B sides. As for the second switch 228, a tracking position error signal 232 detected by a tracking position detector 234 and indicating a positional error of a beam spot 209 for a track is supplied to the second power amplifier 229 through a third filter circuit 233 for phase compensation, a lens access control circuit 235 and the second switch 228. The power amplifier 229 drives the lens actuator 207 on the basis of the tracking position error signal 232. As a result, the lens movable part 208 is located so that the light spot 209 is positioned onto the nearest track (not shown).
As for the third switch 238, the lens position signal 226 detected by the lens position detector 206 is supplied to the first power amplifier 221 through a fourth filter circuit 237 for phase compensation to control the optical head 201 through the head actuator 212 so that the optical head 201 is located into a position at which the lens position signal 226 is zero. In this manner, while the light spot 209 is correctly located on a track, the optical head 201 is located into the position at which the lens position signal is zero (that is, the lens movable part 208 is located into a center position in a movable range for tracking control). Thereby, the optical head 201 can follow any variation of the track position at a relatively low frequency caused by the eccentricity of the disk 210 and it becomes possible to always locate the lens movable part 208 in the center position into the movable range for tracking control. Accordingly, the accuracy of location or positioning of the optical head 209 into the track position can be greatly improved since the lens movable part 208 is not influenced by a supporting spring (not shown) even if the eccentricity if the disk 210 is large.
If the light spot 209 is thus located into the nearest track, the address of the track on which the light spot 209 is positioned is read by an information reproducing circuit (not shown). In general, a possibility that the light spot 209 may be located into the target track through only one movement of the optical head 201 is little. Therefore, the light spot 209 is moved over the number of tracks corresponding to a difference between the address of the target track and the address of a track on which the light spot 209 is positioned at present. This movement of the light spot 209 is carried out through the repetition of a track jump which usually causes movement over one track. The track jump is made as follows. Namely, a track jump instruction (signal) 236 is externally supplied to the lens access control circuit 235. On the basis of the track jump instruction 236, the lens access control circuit 235 effects the track jump of the light spot 209 through the second switch 228, the second power amplifier 229 and the lens actuator 207 so that the lens movable part 208 is moved over one track or the light spot 209 is located into the next or adjacent track.
Though the output signals of the head position detector 213, the lens position detector 206 and the tracking position detector 234 are amplified by respective amplifiers, the illustration and explanation thereof are omitted from FIG. 11 and the above description.
As a second prior art is known an optical recording and reproducing system which is described by JP-A-62-289929 and has a construction shown in FIG. 12.
In FIG. 12, reference numeral 1 designates a disk, numeral 2 an optical head, numeral 3 a linear motor, numeral 4 a light emitting element, numeral 5 an optical position detector, numeral 6 a moving part driving circuit, numeral 31 a detecting circuit, numeral 32 a subtracting circuit, numeral 34 a digital-to-analog (D/A) converting circuit, numeral 35 an operating circuit, and numeral 36 a memory. In this second prior art, the overall detection range of the optical position detector 5 is bisected, thereby improving the linearlity of an output signal of the detector.
First, in order to move the optical head 2 to a track (not shown) in the inner circumferential portion of a recording area of the disk 1, a sensor position signal 61' corresponding to the inner circumferential position is supplied from the operating circuit 35 to the subtracting circuit 32 through the D/A converting circuit 34. When the sensor position signal 61' is outputted, the linear motor 3 is moved to the designated inner position by virtue of a position feedback system which includes the subtracting circuit 32, the movable part driving circuit 6, the linear motor 3, the light emitting element 4, the optical position detector 5 and the detecting circuit 31. Next, a track signal 60 at the inner circumferential position outputted from the optical head 2 is demodulated to track identification signal 62 by a data detecting circuit 38 and an identification circuit 37. A track identification signal 62 obtained is supplied to the operating circuit 35. And, the track demodulation signal 62 is stored into the memory 36. At the same time, a sensor position signal 61 (a digital version of the signal 61') corresponding to the inner circumferential position is also stored into the memory 36.
The above-mentioned consecutive operation is similarly carried out also for each of the intermediate and outer circumferential portions of the recording area of the disk 1.
At the point of time when the above operation has been completed three times, data including three kinds of sensor position signals 61 and three kinds of corresponding track identification signals 62 at the inner, intermediate and outer circumferential positions are stored in the memory 36. Though the track signal 60 and the sensor position signal 61' are amplified by respective amplifier circuits, the illustration and explanation thereof are omitted from FIG. 12 and the above description.
Next, the procedure of control in the second prior art will be explained referring to a flow chart shown in FIG. 13.
Though the above-mentioned three operation procedures A, B and C at the inner, intermediate and outer circumferential positions, there can be obtained a sensor position signal S.sub.a and a track identification signal T.sub.a at the inner circumferential position A, a sensor position signal S.sub.b and a track identification signal T.sub.b at the intermediate circumferential position B and a sensor position signal S.sub.c and a track identification signal T.sub.c at the outer circumferential position C. These signals S.sub.a to S.sub.c and T.sub.a to T.sub.c are stored in the memory 36. Next, a proportional constant K.sub.ab between a sensor position signal and a track identification signal in a range from the inner circumferential position A to the intermediate circumferential position B is determined by EQU K.sub.ab =(S.sub.b -S.sub.a)/(T.sub.b -T.sub.a).
The value of K.sub.ab is calculated and the determined value is stored into the memory 36 (steps 50 and 51).
Similarly, a proportional constant K.sub.bc between a sensor position signal and a track identification signal in a range from the intermediate circumferential position B to the outer circumferential position C is determined by EQU K.sub.bc =(S.sub.c -S.sub.b)/(T.sub.c -T.sub.b)
and the determined value of K.sub.bc is stored into the memory 36 (steps 52 and 53).
In this manner, the overall detection range of the optical position detector 5 is bisected, thereby providing two approximated linear characteristics K.sub.ab and K.sub.bc.
Next, the procedure of control in the case where a target or desired track identification signal T is supplied will be explained referring to FIG. 14.
In FIG. 14, whether or not the target track identification signal T inputted at step 71 is greater than the track identification signal T.sub.b at the intermediate circumferential position is judged at step 71. If the target track identification signal T is equal to or smaller than the track identification signal T.sub.b, which means that the target track is inside of the intermediate circumferential position, the proportional constant K.sub.ab is used and a sensor position signal S to be outputted is determined as being EQU S=K.sub.ab (T-T.sub.a)+S.sub.a
(step 73). On the other hand, if the target track identification signal T is greater than the track identification signal T.sub.b at the intermediate circumferential position, which means that the target track is outside of the intermediate circumferential position, the proportional constant K.sub.bc is used and a sensor position signal S to be outputted is determined as being EQU S=K.sub.bc (T-T.sub.b)+S.sub.b
(step 74).
The sensor position signal S thus determined is supplied from the operating circuit 35 to the D/A converting circuit 34 (step 75).
However, these prior arts have the following problems.
(1) Since the linear motor is moved in a so-called speed or velocity control manner in the case where access to the target track is to be made, the number of circuit components required is increased. For example, a detector for detection of the speed or velocity of the linear motor or a subtracting circuit for subtraction from a target or desired velocity signal is required. Also, a follow-up delay determined by a servo band of the feedback system is large.
(2) An error in the accuracy of positioning or location of the linear motor is produced in accordance with the eccentricity of the disk.
(3) It is necessary to correct the linear characteristic of an output signal of the optical position detector each time the disk is interchanged.
(4) An error in the accuracy of positioning of the linear motor is produced due to a change in characteristic of the optical position detector resulting from the variation of temperature or the lapse of time.